11.12.2017 Research publication
Critical behavior of the two-dimensional icosahedron model

In the context of a discrete analog of the classical Heisenberg model, we investigate the critical behavior of the icosahedron model, where the interaction energy is defined as the inner product of neighboring vector spins of unit length pointing to the vertices of the icosahedron. The effective correlation length and magnetization of the model are calculated by means of the corner-transfer-matrix renormalization group (CTMRG) method. A scaling analysis with respect to the cutoff dimension m in CTMRG reveals a second-order phase transition characterized by the exponents ν=1.62±0.02 and β=0.12±0.01. We also extract the central charge from the classical analog of entanglement entropy as c=1.90±0.02, which cannot be explained by the minimal series of conformal field theory.

06.11.2017 Popularisation
Žiadne jablko nepadlo kvantovému fyzikovi do cesty|| &downarrow;&downarrow;&downarrow; ||
An interview about his way to quantum technologies with Mario Ziman for Eductech in the series Road to profession.
|| ΞΞΞ || (in Slovak)

An alternative model for a description of magnetization processes in coupled 2D spin-electron systems has been introduced and rigorously examined using the generalized decoration-iteration transf ormation and the corner transfer matrix renormalization group method. The model consists of localized Ising spins placed on nodal lattice sites and mobile electrons delocalized over the pairs of decorating sites. It takes into account a hopping term for mobile electrons, the Ising coupling between mobile electrons and localized spins as well as the Zeeman term acting on both types of particles. The ground-state and finite-temperature phase diagrams were established and comprehensively analyzed. It was found that the ground-state phase diagrams are very rich depending on the electron hopping and applied magnetic field. The diversity of magnetization curves can be related to intermediate magnetization plateaus, which may be continuously tuned through the density of mobile electrons. In addition, the existence of several types of reentrant phase transitions driven either by temperature or magnetic field was proven.

05.10.2017 News
Quantum communication security between Beijing and Vienna (7400 km)Last Friday (29/09/2017) the chinesse satelite Micius (launched in 2016) enabled us to share a secret key in the quantum way. Researchers from Austria (led by Anton Zeilinger) and China (led by Jian-Wei Pan) exploits the quantum nature of photon's polarisation to implement the first space-based quantum key distribution protocol. This makes the era of quantum internet a (qu)bit closer. Congratulations! ||ΞΞΞ|| and || video ||

04.10.2017 News
Physics Nobel Prize 2017 for observation of gravitational waves.
For their decisive contributions to the LIGO detector and the observation of gravitational waves the Prize is awarded to Rainer Weiss, Barry C. Barish and Kip Thorne. Gravitational waves were predicted hundred years ago by Albert Einstein. LIGO experiment has observed gravitational waves that were created in the collision of two black holes more than 1.3 bilions years ago, i.e. when first plants started to inhabiqtate our Earth.

Although quantum physics was founded more than ninety years ago this very useful and successful theory still seems unintuitive and puzzling to everyone. Devices relying on quantum effects already found their way into our everyday lives (Lasers, LED lights, Magnetic Resonance Imaging, …), yet they have not changed much our thinking. However, the thoughts of modern physics like R. Feynman, D. Deutsch and Peter Shor may revolutionize the way we think about information and computation. The aim of this talk is to describe basic ideas behind advantages of quantum computers and to explain how they ruin common cryptographic schemes, but gave us unconditionally secure quantum cryptography.

02.10.2017
Our Researcher's Night 2017

29.09.2017 Popularisation activity
The world as the computation
(in Slovak, Svet ako výpočet)
ΞΞΞ
During the festival Researcher's Night 2017 happening
on September 29th (20:30, Stará tržnica, Bratislava) our
colleague Daniel Nagaj will be giving a magical talk
on the beauty of complexity theory dressed in quantum settings.
You will learn about quantum teleportation and related subjects
that has inspired the so-called second quantum revolution in science
and development of future quantum technologies.

29.09.2017 Popularisation activity
Quantum world
(in Slovak, Kvantový svet) ΞΞΞ
Short 15 minutes introduction to quantum magic simulated by kinetic balls and presented by Mario Ziman (or other way round?). The presentations are part of Resercher's Night Fest in Stará Tržnica (Bratislava). Be there
at 9:15, 12:00 and 15:00 and look for the main stage.

21.09.2017 Popularisation activity
Meeting us at Researcher's night in Bratislava
Book Friday, September 29th for a closer quantum experience
of the second type. We will be there in Stará tržnica in Bratislava
happy to introduce you to the world of quantum phenomena and to explain
the bright future of quantum technologies. Under our guidiance you
could try to touch the qubit, or to experience the quantum car drive,
or to compete in math with the quantum computer.
You are welcome to visit our stand to really understand
otherwise boring performance of kinetic balls, or join our presentations
at the main stage

22.09.2017 Research publication
Higher-order spin and charge dynamics in a quantum dot-lead hybrid system

Understanding the dynamics of open quantum systems is important and challenging in basic physics and applications for quantum devices and quantum computing. Semiconductor quantum dots offer a good platform to explore the physics of open quantum systems because we can tune parameters including the coupling to the environment or leads. Here, we apply the fast single-shot measurement techniques from spin qubit experiments to explore the spin and charge dynamics due to tunnel coupling to a lead in a quantum dot-lead hybrid system. We experimentally observe both spin and charge time evolution via first- and second-order tunneling processes, and reveal the dynamics of the spin-flip through the intermediate state. These results enable and stimulate the exploration of spin dynamics in dot-lead hybrid systems, and may offer useful resources for spin manipulation and simulation of open quantum systems.

The divisibility of dynamical maps is visualized by trajectories in the parameter space and analyzed within the framework of collision models. We introduce ultimate completely positive (CP) divisible processes, which lose CP divisibility under infinitesimal perturbations, and characterize Pauli dynamical semigroups exhibiting such a property. We construct collision models with factorized environment particles, which realize additivity and multiplicativity of generators of CP divisible maps. A mixture of dynamical maps is obtained with the help of correlated environment. The mixture of ultimate CP divisible processes is shown to result in a class of eternal CP indivisible evolutions. We explicitly find collision models leading to weakly and essentially non-Markovian Pauli dynamical maps.

13.09.2017 Research publication
Finding paths with quantum walks or quantum walking through a maze

We show that it is possible to use a quantum walk to find a path from one marked vertex to another. In the specific case of M stars connected in a chain, one can find the path from the first star to the last one in O(M√N) steps, where N is the number of spokes of each star. First we provide an analytical result showing that by starting in a phase-modulated highly superposed initial state we can find the path in O(M√N logM) steps. Next, we improve this efficiency by showing that the recovery of the path can also be performed by a series of successive searches when we start at the last known position and search for the next connection in O(√N) steps leading to the overall efficiency of O(M√N). For this result we use the analytical solution that can be obtained for a ring of stars of double the length of the chain.

18.08.2017 Research publication
Nuclear-spin-induced localization of edge states in two-dimensional topological insulators

We investigate the influence of nuclear spins on the resistance of helical edge states of two-dimensional topological insulators (2DTIs). Via the hyperfine interaction, nuclear spins allow electron backscattering, otherwise forbidden by time-reversal symmetry. We identify two backscattering mechanisms, depending on whether the nuclear spins are ordered or not. Their temperature dependence is distinct but both give resistance, which increases with the edge length, decreasing temperature, and increasing strength of the electron-electron interaction. Overall, we find that the nuclear spins will typically shut down the conductance of the 2DTI edges at zero temperature.

31.05.2017 workshop
Central European Quantum Information Processing 2017CEQIP workshop is traditionally focused on current challenges and paradigms of mathematical and computational aspects of emerging quantum technologies. Thanks all for coming and looking forward to see you next year. :: CEQIP website ::

11.04.2017 seminar
QUTE SLOVAKIA
In connection with the preparation of European Commission project "Quantum Flagship" the Presidum of Slovak Academy of Sciences is organizing an informal seminar aiming to introduce this project for Slovak researchers, to formulate our national priorities within the area of quantum technologies and to outline Slovak-wide supporting innitiative in the area of quantum technologies.

In the exact quantum query model a successful algorithm must always output the correct function value. We investigate the function that is true if exactly k or l of the n input bits given by an oracle are 1. We find an optimal algorithm (for some cases), and a nontrivial general lower and upper bound on the minimum number of queries to the black box.